Procedure for setting laser and heater power in HAMR device

ABSTRACT

A heater power of a heat-assisted magnetic recording head is set to an initial power to induce an initial head-medium clearance. For a plurality of iterations, a heater power at an optimum laser power is determined that achieve a target clearance. If differences in the heater power and optimum laser power between the two subsequent iterations are below a threshold, the iterations are stopped and the heater power and the optimum laser power for one of the two subsequent iterations is used as an operational heater power and laser power for the heat-assisted magnetic recording head.

SUMMARY

The present disclosure is directed to setting of laser and heater powerin a heat-assisted magnetic recording device. In one embodiment, amethod involves setting a heater power of a heat-assisted magneticrecording head to an initial power to induce an initial head-mediumclearance. For a plurality of iterations, a laser power of the recordinghead is varied while writing data to at least one track of a recordingmedium at the initial head-medium clearance. The iterations also involvedetermining an optimum laser power based on reading the data; applyingan additional heater power to approach or cause a head-medium contact atthe optimum laser power; and setting the heater power for a nextiteration based on an offset from the additional heater power. If afirst difference in the heater power between two subsequent iterationsis below a first threshold and a second difference in the optimum laserpower between the two subsequent iterations is below a second threshold,the iterations are stopped and the heater power and the optimum laserpower for one of the two subsequent iterations is used as an operationalheater power and an operational laser power for the heat-assistedmagnetic recording head.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below makes reference to the following figures, whereinthe same reference number may be used to identify the similar/samecomponent in multiple figures.

FIG. 1 is a block diagram of a hard drive slider and media arrangementaccording to an example embodiment;

FIG. 2 is a cross-sectional view of a read/write head according to anexample embodiment;

FIG. 3 is a flowchart of a procedure according to an example embodiment;

FIG. 4 is a graph showing results of a calibration procedure accordingto an example embodiment;

FIG. 5 is a block diagram of a system and apparatus according to anexample embodiment; and

FIG. 6 is a flowchart of a method according to an example embodiment.

DETAILED DESCRIPTION

The present disclosure generally relates to detection and control ofhead-media spacing in data storage devices and the setting of theoperational laser power in a heat assisted magnetic recording device(HAMR). The detection of head-to-media spacing becomes more challengingin what are referred to as heat-assisted magnetic recording devices.This technology, also referred to as energy-assisted magnetic recording(EAMR), thermally-assisted magnetic recording (TAMR), andthermally-assisted recording (TAR), uses an energy source such as alaser to heat a small spot on a magnetic disk during recording. The heatlowers magnetic coercivity at the spot, allowing a write transducer tochange magnetic orientation. Due to the relatively high coercivity ofthe medium after cooling, the data is less susceptible to paramagneticeffects that can lead to data errors.

Generally, recording heads may utilize heaters for fine control ofhead-to media spacing. The heaters heat a portion of the recording headthat faces the recording medium. The heating causes a local protrusiondue to thermal expansion of the material. Thermal protrusion can befinely controlled to maintain a desired clearance between read/writetransducers and the recording medium. As will be explained in greaterdetail below, the introduction of a HAMR energy source to the read/writehead can complicate the control of head-to-media spacing. Further, whileconventional read/write heads may be allowed to contact the recordingmedium under some conditions, a HAMR device may be damaged if suchcontact occurs while recording. This can make the estimation and controlof head-to-media spacing more difficult in a HAMR recording head.

In reference now to FIG. 1, a block diagram shows a side view of aread/write head 102 according to an example embodiment. The read/writehead 102 may be used in a magnetic data storage device, e.g., harddrive. The read/write head 102 may also be referred to herein as aslider, read head, recording head, etc. The read/write head 102 iscoupled to an arm 104 by way of a suspension 106 that allows somerelative motion between the read/write head 102 and arm 104. Theread/write head 102 includes read/write transducers 108 at a trailingedge that are held proximate to a surface 110 of a magnetic recordingmedium 111, e.g., magnetic disk. When the read/write head 102 is locatedover surface 110 of recording medium 111, a flying height 112 ismaintained between the read/write head 102 and the surface 110 by adownward force of arm 104. This downward force is counterbalanced by anair cushion that exists between the surface 110 and an air bearingsurface (ABS) 103 (also referred to herein as a “media-facing surface”)of the read/write head 102 when the recording medium 111 is rotating.

It is desirable to maintain a predetermined slider flying height 112over a range of disk rotational speeds during both reading and writingoperations to ensure consistent performance. Region 114 is a “closepoint” of the read/write head 102, which is generally understood to bethe closest point of contact between the read/write transducers 108 andthe magnetic recording medium 111, and generally defines thehead-to-media spacing 113. To account for both static and dynamicvariations that may affect slider flying height 112, the read/write head102 may be configured such that a region 114 of the read/write head 102can be configurably adjusted during operation in order to finely adjustthe head-to-media spacing 113. This is shown in FIG. 1 by dotted linethat represents a change in geometry of the region 114. In this example,the geometry change may be induced, in whole or in part, by an increaseor decrease in temperature of the region 114.

To provide dynamic control of head-to-media spacing 113 via heat, theread/write head 102 may include (or otherwise be thermally coupled to)one or more heating elements 116. The heating element 116 (e.g.,resistance heater) may be provided with selectable amounts of power by acontroller 118. An increase or decrease in current causes and increaseor decrease in the temperature of the heating element 116, which resultsin expansion or contraction at the media-facing surface 103.

In addition to controlling the heating element 116, the controller 118includes logic circuitry for controlling other functions of a datastorage apparatus. The data storage apparatus includes at least theread/write head 102 and recording medium 111, and may include othercomponents not shown, such as spindle motor, arm actuator, powersupplies, etc. The controller 118 may include or be coupled to interfacecircuitry 119 such as preamplifiers, buffers, filters, digital-to-analogconverters, analog-to-digital converters, etc., that facilitateelectrically coupling the logic of the controller 118 to the analogsignals used by the read/write head 102 and other components not shown.

Other elements of the read/write head 102 may also provide heat besidesor in addition to the heating element 116. For example, a write coil ofthe read/write transducers 108 may generate sufficient heat to causeconfigurable deformation of region 114. This deformation will only occurwhen the coil is energized, e.g., when data is being written. Further,the illustrated read/write head 102 is configured as a HAMR recordinghead, which includes additional components that generate heat near theread/write transducer 108. These components include laser 120 (or otherenergy source) and waveguide 122. The waveguide 122 delivers light fromthe laser 120 to components near the read/write transducers 108. Thesecomponents are shown in greater detail in FIG. 2.

In FIG. 2, a block diagram illustrates a cross-sectional view of theHAMR read/write head 102 according to an example embodiment. Thewaveguide 122 receives electromagnetic energy 200 from the energysource, the waveguide 122 coupling the energy to a near-field transducer(NFT) 202. The NFT 202 is made of a metal (e.g., gold, silver, copper,etc.) that achieves surface plasmonic resonance in response to theapplied energy 200. The NFT 202 shapes and transmits the energy tocreate a small hotspot 204 on the surface 110 of medium 111. A magneticwrite pole 206 causes changes in magnetic flux near the media-facingsurface 103 in response to an applied current. Flux from the write pole206 changes a magnetic orientation of the hotspot 204 as it moves pastthe write pole 206 in the downtrack direction (z-direction).

The energy 200 applied to the near-field transducer 202 to create thehotspot 204 can cause a significant temperature rise in local region.The near-field transducer 202 may include a heat sink 208 that drawsaway some heat, e.g., to the write pole 206 or other nearbyheat-conductive component. Nonetheless, the temperature increase nearthe near-field transducer 202 can be significant, leading to localprotrusion in the region of the write pole 206 and near-field transducer202.

From head to head, there is may be significant variation in passive flyheight, optical efficiency of the NFT and light path, and othervariations that may affect dynamic fly height response. In such a case,the amount of protrusion from head to head due to thermal effects andthe amount of laser power needed to optimally record will be differentfrom head to head. In FIG. 3, a flowchart shows an example of a methodfor setting the laser power that corrects for these variations andensures that a fixed clearance is maintained at the optimal laser power.

The method represented in FIG. 3 may be performed during qualificationof a HAMR device, e.g., in the factory after manufacture. The method mayalso be performed post-manufacture, e.g., as part of a regularrecalibration process, manually or automatically initiated to mitigateperformance issues, etc. While the following procedure describes thesetting and determination of laser and heater power, this may alsoencompass indirect estimates of power. For example, a controller may setpower levels by changing one or both of a current and a voltage appliedto the heater and laser. Generally, the values may be discrete valuesinput to a digital-to-analog converter (DAC), which converts thediscrete values to an analog voltage or current.

The procedure begins by initializing 300 a counting variable N used insubsequent iterations. An initial estimate of correct reader heaterpower HP_(N) is made at block 302, and a laser power value LP_(N) isinitialized to zero. The initial heater power value HP_(N) may bespecific to certain conditions, such as a zone/region of the recordingmedium being tested, ambient temperature, rotation speed of the medium,etc. Both HP_(N) and LP_(N) will be determined during an iteration, andcompared with corresponding HP_(N+1) and LP_(N+1) in subsequentiterations. As such, the procedure will involve storage of heater andpower values from at least one previous iteration, and values for moreiterations may also be stored, e.g., to calculate measures such as slopethat indicate whether convergence is achievable.

At block 306, a series of data are recorded by sweeping through a laserpower value (e.g., LP₀ to LP_(M)) and then reading back the data thatwas recorded at the different powers. The heater and laser power valuesmay be discrete values input to a DAC, and so may sweeping through thelaser power may involve incrementing a discrete DAC input value. In oneembodiment, the laser power may be swept while recording a single track.In another embodiment, three tracks are written and the middle track isused as the reference track. With this method, the effects of adjacenttrack interference can be accounted for. In another embodiment, a wholetrack (or multiple tracks) can be written at one laser power level, readback, and then the process repeated at a different laser power levelsover the same track(s) or equivalent tracks. As with the heater power,the range of laser power levels used in the sweep may be dependent onzone, temperature, and other factors.

At block 308, an optimal recording laser power is found. This optimalrecording laser power may be based on one or more measurements madewhile reading back the recorded data, such as any combination ofsignal-to-noise ratio (SNR), bit error rate (BER), track averageamplitude, track width, and overwrite performance. For example, if SNRis used, the optimum laser power may correspond to any combination of apeak SNR, target change in slope of SNR, SNR for tracks written atincreasingly smaller pitch (e.g., squeezed SNR), etc. If BER is used,the BER may be measured for an isolated track and/or over multipletracks. In the latter case, the laser power can be chosen thatcorresponds to minimum BER of a main track with two adjacent trackswritten at a fixed pitch to simulate drive operational conditions.

In some cases, neither SNR nor BER may be directly available, and soother measurements may be made and analyzed at block 308. For example,track average amplitude or track width may be used. If track width isused, laser power may be increased until a desired track width isachieved. Track width can be measured by scanning the reader over awritten track, e.g., reading the track at various crosstrack offsetseither side of the track center. The width may be defined, for example,as the 50% amplitude point on both sides of the track centerline. Inanother example where neither SNR nor BER are directly available, servoerror rate may be used instead to determine optimal recording power.Servo error rate is the error rate of the sync mark in tracks which arestitched together during the writing process. This may be considered asingle-sided squeeze metric.

In some systems, like servo copy in a drive, neither SNR nor BER areavailable. With servo copy, a seed pattern is written by a specializedmanufacturing machine. The drive then uses this seed pattern to fill inthe missing servo patterns needed by the drive. In this case, the signalfrom the servo variable gain amplifier (VGA) can be used to determineoptimal recording power. When a servo sync signal is written between theservo marks and user data sectors, the signal is first adjusted usingthe servo automatic gain control plant. A signal which is written weaklyrequires more gain from the VGA. As laser power is increased, less gainis required. The VGA gain can be used to pick the optimal laser power atblock 308, e.g., laser power that corresponds to a minimum value of VGAgain.

In other embodiments, an overwrite signal can be used as a metric fordetermining an optimal laser power. Overwrite generally involves writinga tone at a first frequency and then immediately over write that data ata second, different frequency. When the signal is read it back, it isanalyzed to determine components of each frequency, and in particularhow much of the first frequency can still be read. The quality of therecording is inversely related to how much of the first frequency isstill present, e.g., the amplitude of the first frequency component. Itwill be understood that any available measures described herein todetermine optimum laser power may be used in combination, e.g., eachbeing a weighted contribution to an overall score, the optimum laserpower being the one with the best score.

At block 310, the laser is set to the optimum laser power, which isdesignated as LP_(N+1) to differentiate it from either the initial LP₀or LP_(N) if this is not the first iteration (N>0). Also at this block310, heater is increased until head-to-medium contact is detected, oruntil a suitably small clearance is detected just before contact.Contact can be detected by using an acoustic emission (AE) detector,which detects vibrations resulting from the contact. Other contactand/or clearance detection schemes may be used, such analyzing a thermalprofile detected by a thermal sensor located near the media-facingsurface of the read/write head. In another embodiment, contact and/orclearance can be detected based on position error sensor (PES) signals.For example, when the head contacts the medium, it may skip to the leftor right, resulting in a jump in the PES signal. This block 310 may beperformed in every zone or a subset of zones, and so a contactlessdetection process may be preferred to minimize damage to the read/writehead.

At block 312, heater power is backed off by a particular amount tomaintain the target clearance using the optimum laser power LP_(N+1) forthis iteration. The heater power so obtained is considered an optimumheater power for the iteration and is designated LP_(N+1) for thisiteration. For example, the optimum heater power LP_(N+1) may beobtained by reducing the contact or near-contact heater power by apredetermined amount known to induce a desired negative displacement. Inother cases, the heater power may be gradually reduced while activelymeasuring clearance until the desired clearance is reached. In such acase, a heater power that results in the desired clearance is saved asthe new optimum heater power LP_(N+1).

At block 314, the current optimum heater and laser power are comparedare both compared (e.g., by subtraction, comparison of discrete DACvalues, etc.) to previous optimum heater and laser power, and if theirdifference is greater than a threshold amount (which may be zero), thenthe counter N is incremented at block 316 and the next iteration beginsat block 306. If the difference is less or equal to the threshold, thenthe last used heater and laser power are prepared for use (e.g., storedin memory) for subsequent use in writing data, as indicated at block318.

The procedure shown in FIG. 3 can be performed for each read/write headin a device, and performed over different zones of a recording medium.In a controlled test environment (e.g., qualification testing) otherfactors can also be varied while performing the tests, such as ambienttemperature. For each combination of variables, an optimum heater andlaser power may be determined, either through direct lookup of testresults performed at the corresponding conditions, and/or viaextrapolation of results performed at a subset of the conditions. Theprocedure may be performed during manufacture of the device and/orduring operational use.

In FIG. 4, a graph illustrates results of the example procedureaccording to an example embodiment performed on a number of HAMR harddrive read/write heads. The number of iterations is on the x-axis andthe clearance is on the y-axis. It has been found that, in most cases,the system can reach the optimal combination of laser power and heaterpower in as few as three iterations. The chart shows that initiallythere is a large distribution in writer clearance for each head butafter three iterations of the method described above, nearly all headare writing at a fixed target clearance indicated by line 400. In thesetests SNR was used to determine the optimum laser power during the laserpower sweep part of the test.

In FIG. 5, a block diagram illustrates a data storage system accordingto an example embodiment. A data storage apparatus 500 includes logiccircuitry 502 used to read data from and write data to one or moremagnetic disks 510. The magnetic disks 510 are configured as aheat-assisted magnetic recording medium. The logic circuitry 502includes one or more controllers 504 that perform operations associatedwith storing and retrieving data from the disks 510. The operationsinclude processing read and write commands that originate from a hostdevice 506. The host device 506 may include any electronic device thatcan be communicatively coupled to store and retrieve data from a datastorage device, e.g., a computer, peripheral bus card, etc.

The controller 504 is coupled to a read/write channel 508 that processesdata read from and written to the magnetic disk 510. The read/writechannel 508 generally converts data between the digital signalsprocessed by the controller 504 and the analog signals conducted throughone or more read/write heads 512 (also referred to as a recording head).The read write heads 512 are positioned over the magnetic disk 510 via aservo motor 514 (e.g., voice coil motor) that moves one or more arms 516to which the read/write heads 512 are mounted.

During read and write operations, a heater control circuit 518 sendspower to one or more heaters of the read/write head 512. The heatercontrol circuit 518 may include a DAC, preamplifier, filters, etc., thatcontrol and condition signals send to the reader heaters, which are usedto adjust dynamic HMS between the read/write head 512 and disk 510. Thecontroller 504 may receive feedback signals (not shown) that assist incontrolling the heater, such as temperature readings from a head-mountedthermal sensor, AE detection, etc.

During write operations, a laser control circuit 520 sends power to oneor more lasers (or similar thermal energy producing devices) of theread/write head 512. The laser control circuit 520 may include a DAC,preamplifier, filters, etc., that control and condition signals send tothe lasers, which are used energize a near-field transducer that createsa hotspot on the disk 510 during recording. The controller 504 mayreceive feedback signals (not shown) that assist in controlling thelaser, such as intensity readings from a head-mounted photodiode, etc.The laser control circuit 520 may adjust laser power to different levelsduring writing. For example, when traversing servo marks on the disk510, the laser may be kept at a bias current that keeps the laser activebut does not cause the laser to emit enough energy to heat the disk 510to the Curie temperature, thereby preventing corruption of data storedin the servo marks.

The controller 504 may access a persistent storage to accessinstructions and data used in operating the apparatus 500. Thepersistent storage may include any combination of the primary storagemedium (the disk 510 in this case) and local non-volatile solids-statedata storage media, such as flash memory. One example of instructionsand data that may be stored is represented by calibration module 522.

The calibration module 522 includes instructions that cause thecontroller 504 to perform a calibration procedure. The procedureinvolves setting (e.g., via the heater controller 518) a heater power ofthe heat-assisted magnetic recording head 512 to an initial power.Thereafter, a plurality of iterations are performed on the data storageapparatus 500. The iterations involve varying a laser power of therecording head 512 while writing data to at least one track of arecording medium 510 at the heater power. An optimum laser power isdetermined based on reading the data. During each iteration, anadditional heater power is applied to cause a head-medium contactclearance at the optimum laser power. A heater power for the nextiteration is set based on an offset from the additional heater power.

During the iterations, a first difference in the heater power isdetermined between two subsequent iterations. A second differencebetween the optimum laser power between two subsequent iterations alsodetermined. If the first difference is below a first threshold and thesecond difference is below a second threshold, the iterations arestopped. The heater power and the optimum laser power for the lastiteration are used as an operational heater power and an operationallaser power for the heat-assisted magnetic recording head 512. Thevalues of the operational heater power and the operational laser powermay be stored on the apparatus 500, e.g., in non-volatile data storage.It will be understood that some or all of the instructions that causethe controller to perform the calibration procedure may be provided froman external source. For example, the host 506 may be configured as atesting device that directs the calibration procedure as part ofqualification testing.

In reference now to FIG. 6, a flowchart illustrates a method accordingto an example embodiment. The method involves setting 600 a heater powerof a heat-assisted magnetic recording head to an initial power to inducean initial head-medium clearance. A plurality of iterations areperformed, as indicated by block 601. For each iteration, a laser powerof the recording head is varied 602 while writing data to at least onetrack of a recording medium at the initial head-medium clearance. Anoptimum laser power is determined 603 based on reading the data. Theoptimum laser power may be determined 603 based on any combination of: asignal-to-noise ratio of the at least one track; a bit-error rate of theat least one track; a bit-error rate of multiple adjacent tracks; aservo error rate of the at least one track a servo variable gainamplifier gain while reading back the at least one track; a width of theat least one track; and an amplitude of the at least one track.

An additional heater power is applied 604 to approach or cause ahead-medium contact at the optimum laser power. In a case where theadditional heater power causes the head-medium contact at the optimumlaser power, the detecting the head-medium contact may be based onacoustic emissions resulting from the head-medium contact. In a casewhere the wherein the additional heater power causes and approach to thehead-medium contact (but does not cause contact) at the optimum laserpower, the additional heater power may be based on a head-to-mediumclearance that is detected before the head-medium contact occurs. Ineither case, the heater power is set 605 for the next iteration based onan offset from the additional heater power.

It is determined 606 whether a first difference in the heater powerbetween two subsequent iterations is below a first threshold and alsodetermined 607 whether a second difference in the optimum laser powerbetween the two subsequent iterations is below a second threshold. Ifboth determinations 606, 607 are positive, the iterations stop and theheater power and the optimum laser power for one of the two subsequentiterations are used 608 respectively as an operational heater power andan operational laser power for the heat-assisted magnetic recordinghead.

The various embodiments described above may be implemented usingcircuitry and/or software modules that interact to provide particularresults. One of skill in the computing arts can readily implement suchdescribed functionality, either at a modular level or as a whole, usingknowledge generally known in the art. For example, the flowchartsillustrated herein may be used to create computer-readableinstructions/code for execution by a processor. Such instructions may bestored on a non-transitory computer-readable medium and transferred tothe processor for execution as is known in the art.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

The foregoing description of the example embodiments has been presentedfor the purposes of illustration and description. It is not intended tobe exhaustive or to limit the embodiments to the precise form disclosed.Many modifications and variations are possible in light of the aboveteaching. Any or all features of the disclosed embodiments can beapplied individually or in any combination are not meant to be limiting,but purely illustrative. It is intended that the scope of the inventionbe limited not with this detailed description, but rather determined bythe claims appended hereto.

What is claimed is:
 1. A method comprising: setting a heater power of aheat-assisted magnetic recording head to an initial power to induce aninitial head-medium clearance; for a plurality of iterations: varying alaser power of the recording head while writing data to at least onetrack of a recording medium at the initial head-medium clearance;determining an optimum laser power based on reading the data; applyingan additional heater power to approach or cause a head-medium contact atthe optimum laser power; and setting the heater power for a nextiteration based on an offset from the additional heater power; wherein,if a first difference in the heater power between two subsequentiterations is below a first threshold and a second difference in theoptimum laser power between the two subsequent iterations is below asecond threshold, stopping the iterations and using the heater power andthe optimum laser power for one of the two subsequent iterations as anoperational heater power and an operational laser power for theheat-assisted magnetic recording head.
 2. The method of claim 1, whereinthe optimum laser power is determined based on a signal-to-noise ratioof the at least one track.
 3. The method of claim 1, wherein the optimumlaser power is determined based on a bit-error rate of only one track.4. The method of claim 1, wherein the optimum laser power is determinedbased on a bit-error rate of multiple adjacent tracks.
 5. The method ofclaim 1, wherein the optimum laser power is determined based on a servoerror rate of the at least one track.
 6. The method of claim 1, whereinthe optimum laser power is determined based on a servo variable gainamplifier gain while reading back the at least one track.
 7. The methodof claim 1, wherein the optimum laser power is determined based on awidth of the at least one track.
 8. The method of claim 1, wherein theoptimum laser power is determined based on an amplitude of the at leastone track.
 9. The method of claim 1, wherein the optimum laser power isdetermined based on an amplitude of an overwrite signal.
 10. The methodof claim 1, wherein the additional heater power causes a small clearancebeing detected without head-medium contact at the optimum laser power,the method further comprising detecting the clearance based on a thermalprofile at a media-facing surface of the recording head.
 11. A apparatuscomprising: a controller configured to control a heat-assisted magneticrecording head, the controller configured to perform a calibrationprocedure that involves: setting a heater power of the recording head toan initial power to induce an initial head-medium clearance; for aplurality of iterations: varying a laser power of the recording headwhile writing data to at least one track of a recording medium at theinitial head-medium clearance; determining an optimum laser power basedon reading the data; applying an additional heater power to approach orcause a head-medium contact at the optimum laser power; and setting theheater power for a next iteration based on an offset from the additionalheater power; wherein, if a first difference in the heater power betweentwo subsequent iterations is below a first threshold and a seconddifference between the optimum laser power between the two subsequentiterations is below a second threshold, stopping the iterations andusing the heater power and the optimum laser power for one of the twosubsequent iterations as an operational heater power and an operationallaser power for the heat-assisted magnetic recording head.
 12. Theapparatus of claim 11, wherein the optimum laser power is determinedbased on a signal-to-noise ratio of the at least one track.
 13. Theapparatus of claim 11, wherein the optimum laser power is determinedbased on a bit-error rate of only one track.
 14. The apparatus of claim11, wherein the optimum laser power is determined based on a bit-errorrate of multiple adjacent tracks.
 15. The apparatus of claim 11, whereinthe optimum laser power is determined based on a servo error rate of theat least one track.
 16. The apparatus of claim 11, wherein the optimumlaser power is determined based on a servo variable gain amplifier gainwhile reading back the at least one track.
 17. The apparatus of claim11, wherein the optimum laser power is determined based on a width ofthe at least one track.
 18. The apparatus of claim 11, wherein theoptimum laser power is determined based on an amplitude of the at leastone track.
 19. The apparatus of claim 11, wherein the additional heaterpower causes the head-medium contact at the optimum laser power, thecalibration procedure further comprising detecting the head-mediumcontact based on acoustic emissions resulting from the head-mediumcontact.
 20. The apparatus of claim 11, wherein the additional heaterpower approaches the head-medium contact at the optimum laser powerwithout contact occurring, the calibration procedure further comprisingdetecting a head-to-medium clearance via a thermal sensor before thehead-medium contact occurs.